1. Field of the Invention
The invention is in the field of coherent detection such as is used in optical communication systems. More particularly, it relates to an optical mixing device having one photodetector for a heterodyne receiver and a heterodyne receiver provided with such an optical mixing device.
2. Prior art
In a coherent optical heterodyne receiver, a received optical signal is mixed in a mixing device with an optical signal originating from a local oscillator. In this connection, it is a known problem that both the local oscillator signal and the received optical signal exhibit unwanted amplitude variations which are referred to as relative intensity noise (RIN). A first procedure for suppressing this intensity noise is known as balanced detection. A heterodyne receiver in which such balanced detection is used is disclosed, for example, in references [1] and [2]. According to the procedure disclosed therein, the two output signals from a combined coupler/beam splitter are detected by separate photodetectors and then, in a downstream preamplifier circuit, are either first amplified separately and then subtracted or first added in antiphase and then amplified. In this case, use is made of the fact that, at the outputs of such a coupler/beam splitter, the coherent mixing products are in antiphase. Since the intensity noise at the two outputs of the mixing device is equal, it is therefore suppressed, if not completely, nevertheless appreciably. In addition, an intermediate-frequency signal is obtained which is twice as strong in terms of intensity.
Such a balanced detection has the advantage that intensity-noise suppression is obtained without an appreciable loss of signal power. It has the drawback, however, that it is based on two photodetectors which, possibly including associated preamplifiers, must be to a large extent identical to one another, which implies a quality requirement having a strong cost-increasing effect.
A second procedure for intensity-noise suppression, as disclosed in references [3] and [4], eliminates the drawback mentioned. According to this second procedure, at least a portion of the total signal available at the output side of the coupler/beam splitter is split into two signal components having equal power and mutually perpendicular polarisations. Said two signal components are fed to one and the same photodetector, however, in such a manner that they arrive there as one light signal beam but with a mutual phase difference as a consequence of a difference in the optical path length. Said phase difference is chosen such that, at the photodetector, the intermediate-frequency mixing products in the two signal components are in phase, i.e. reinforce one another, whereas the noise components, at least in a relevant region around the intermediate frequency, the intermediate-frequency band, are precisely in antiphase and therefore extinguish one another. Signals outside the intermediate-frequency band are filtered out with electrical filters. The procedure in accordance with reference [3] achieves this by feeding the light signal available at one output of the coupler/beam splitter via a section of highly birefringent (hi-bi) glass fiber having a suitably chosen length to the photodetector. The drawback of this is that only half of the signal power available at the output of the coupler/beam splitter is used and that, in addition, a section of relatively expensive hi-bi fiber is necessary. The procedure according to reference [4] is explained by reference to FIG. 1. According to this known procedure, an optical input signal E.sub.S and an optical local-oscillator signal E.sub.L are fed with mutually identical polarisations to the inputs 1 and 2 of a 3 dB beam splitter 3. Both essentially identical output signals (E.sub.S +E.sub.L) from the 3 dB beam splitter 3 are then fed along separate light paths 4 and 5, via reflecting mirrors 6 and 7 and directly, respectively, to inputs 8 and 9 of a `polarising` (or polarisation-sensitive) beam splitter 10, an output 11 of which is directed at a photodetector 12. Incorporated in one of the light paths, in this case light path 4, is a .pi./2 polarisation rotator 13, with the result that both light signals reach the polarising beam splitter 10 with mutually orthogonal polarisations and emerge via the output 11 as one signal beam. Although this known procedure provides a mixing device having only one photodetector, which provides the same `performance` as the balanced-detection procedure described above and involving two photodetectors, it nevertheless has the drawback that the mixing device requires polarisation-dependent and polarisation-sensitive components and that, as a consequence thereof, an optical fiber construction or an integrated construction of the mixing device is not easy to achieve.